1. Field of Invention
The present invention relates to celestial tracking apparatuses, specifically a wind resistant structure for celestial tracking apparatuses.
2. Prior Art
There is substantial prior art relating to the design and configuration of celestial tracking apparatuses. For example, U.S. Pat. Nos. 3,059,889 (Pottmeyer), 4,870,949 (Butler), and 6,058,930 (Shingleton) all show different configurations of one and two axis celestial tracking apparatuses.
These apparatuses are generally designed to orient a payload, such as a directional radio frequency antenna, a light reflective surface, or a solar energy collection device, toward celestial objects. In the case of a radio antenna, the celestial object could be the sun, a star or planet, or a man-made orbiting satellite. In the case of a light reflective surface, such systems are typically oriented to reflect solar radiation toward an energy collection device. In the case of solar energy collection devices, the system typically orients the payload such that the collection surface minimizes the angle between the axis normal to the device and the solar radiation.
For solar energy collection systems in general, the payload can be one of a number of different energy collection or reflection devices. These can include, among other things, photovoltaic (PV) modules, concentrating PV modules, or concentrating solar thermal devices.
PV modules, or flat plate PV modules as they are also known, generally include one or more planar devices that convert solar radiation into electricity by way of the photoelectric effect. PV modules are the dominant technology in solar energy collection systems.
Concentrating PV modules also use the photoelectric effect to produce electricity, but the modules use refraction, reflection, or some other optical technique to concentrate the incident solar radiation onto a PV device. Concentrating PV modules are generally of a thicker form factor than flat plate PV modules. Some examples of prior art relating to concentrating PV modules are found in U.S. Pat. Nos. 4,834,805 (Erbert) and 6,559,371 (Shingleton).
Besides modules, other concentrating PV form factors are also found in the prior art. A typical example is a paraboloidal or spherical reflector that focuses solar radiation at a PV device positioned at or near the focal point. An example of prior art for this configuration is found in U.S. Pat. No. 4,234,352 (Swanson). In some cases, the solar radiation is captured by a thermal device, such as a stirling engine or steam generator, which is coupled to an electric generator. The thermal device can be part of the payload, as is shown in U.S. Pat. No. 4,583,520 (Dietrich).
In yet another concentrating configuration the tracking apparatus is a heliostatic device, upon which a reflective payload is mounted. The energy collection device is mounted remotely, and is typically a solar thermal device as described above. An example of a tracking apparatus used in this type of configuration is shown in U.S. Pat. No. 7,115,851 (Zhang).
In general, celestial tracking apparatuses include one or more axes of motion, the function of which is to allow the collector to be oriented as described above. In the theoretically ideal case, two axes of motion are sufficient to orient the collector to face in any desired direction. Practically speaking, mechanical considerations can limit the range of motion such that the tracking apparatus might not be able to address all arbitrary points in the sky. However, in many applications, such as solar energy collection systems, the range of desired orientations is also limited to a subset of the visible sky. For instance, in the northern hemisphere below the arctic circle, a due North orientation is unlikely to be desirable, since the sun is never located in that direction.
As mentioned previously, a two axis tracking apparatus is theoretically sufficient for perfect tracking of a celestial object. However, other practical considerations have resulted in different configurations in the prior art. In solar energy collection systems, such as those using flat plate PV modules, precise orientation relative to the sun is not always critical. This is because small deviations from a module orientation normal to the direction of incident solar radiation results in only small reductions in energy collection. As such, single axis tracking apparatuses, which are generally not capable of orienting the energy collection device perfectly normal to the solar radiation, are commonly used in solar PV systems.
Further, dual axis designs may not have the dynamic range required to address a large portion of the sky while maintaining the high accuracy required for high concentration PV modules or for telescope applications. As such, additional axes may be required in some cases to trim the orientation of the collector to more precisely track the celestial object.
Celestial tracking apparatuses can be installed in any number of locations, but in general the installation can be classified as either ground mounted or structure mounted. A ground mounted tracking apparatus is one that is installed in direct contact with the ground. Ground mounted systems often include a foundation as the interface between the apparatus and the ground. Ground mounted systems also include tracking apparatuses that are installed on existing concrete slabs or other similar situations. Structure mounted tracking apparatuses are those that are installed on an existing structure, or those that are attached to a structure that fulfills a separate purpose. For example, a tracking apparatus can be installed on the roof of an existing building, or can be constructed as part of a parking structure or carport.
Regardless of the type, all celestial tracking apparatuses must be designed to resist a set of design loads. Depending on the specific circumstances of the installation, these loads can include the dead load, or load due to gravity, wind loads, snow loads, and seismic loads. In many cases, the wind load is the dominant factor in tracker design.
While not always the case, the wind is a substantially horizontal phenomenon, and as such tends to impart lateral loads. Among other effects, lateral loads acting above the base of the apparatus tend to impart an overturning moment. Apparatus designs found in prior art resist overturning moments through the use of a foundation. In the prior art, the foundation is generally responsible for providing overturning moment resistance and can generally be divided into two categories. One type can be categorized as a dug-in foundation, which means that the base of the structure extends substantially into the ground. A typical example of such a system can be viewed in the Installation Instructions for the Wattsun AZ-225 Tracking System from Array Technologies. With a dug-in foundation, wind loads that act to overturn the structure are resisted by lateral forces between the soil and the foundation. While this is a well established and understood foundation type that effectively resists wind loads, it also has some disadvantages. Primary among these is the additional cost of excavation required to insert the foundation into the ground. Another consideration with dug-in foundations is that variability in soil conditions can make it difficult to predict how much resistance to overturning the foundation can provide without measuring soil conditions at the precise location where the foundation will be placed.
A second category of established wind resistant foundation design is ballasted foundations. Ballasted foundations generally use a mass of fixed material, such as concrete, as a base for the tracking apparatus. Ballasted foundations typically rest directly on the ground or extend only slightly below the surface, and rely primarily on gravitational forces acting on the foundation to prevent the structure from overturning. The Solon Mover sold by Solon AG of Germany is one of several example of prior art that use a ballasted foundation. While this type of foundation reduces the cost and variability concerns associated with the dug-in foundations mentioned above by reducing the excavation costs, the amount of material required to provide sufficient mass for the foundation can result in an inefficient use of materials.
Note that although the prior art examples described above primarily show two axis tracking apparatuses, both dug-in foundations and ballasted foundations are prevalent in single axis tracking apparatus prior art as well. As an example if a dug-in foundation, in U.S. Pat. No. 6,058,930, Shingleton describes a single axis tracking apparatus that is mounted on footings of poured concrete supported in the Earth. SunPower Corporation's T20 Tracker, which is a tilted axis single axis tracker (http://sunpowercorp.com/Products-and-Services/Trackers.aspx), uses three ballasted foundations as the bases for a tripod-like structure.
While wind resistant tracking apparatus foundation designs are both well known and effective, these designs are by no means optimal. In particular, traditional foundation designs are costly, with the cost of materials and installation for tracking apparatus foundations typically representing approximately 25% or more of total mounting system cost. In cost sensitive applications such as solar energy systems cost reduction is always desirable, but the prospects of cost reduction for prior art designs are limited. This is particularly the case for ballasted foundation designs, because the minimum mass of foundation material required to prevent overturning is fixed by the apparatus design and thus can not be further reduced.
For dug-in foundations, the problems associated with the unpredictability of soil property variations also appears difficult to solve. Further, the long term trend in costs associated with the intensive labor needed for the excavation required for such foundations appears to be upward. As such, it does not appear that significant reductions in foundation costs are likely within the scope of traditional designs.
Despite the many variations of tracking apparatus configurations described above, the materials used to build the structures of these systems are generally the same. In nearly all cases where the system is ground mounted, the tracking apparatus contacts the Earth through a foundation. In many cases, the foundation is made of reinforced concrete placed at or below grade in such a manner as to resist loads on the system and to distribute those loads over a sufficiently large area of soil.
The structure above the foundation, on which the PV modules are mounted and which comprises the tracking mechanism are generally made of metal. As tracking apparatuses are usually exposed to the elements over long design lifetimes of 25 years or more, it is desirable to use materials that are corrosion resistant. The most common materials used in these applications are painted, epoxy coated, or galvanized steel and/or aluminum. While these materials can be sufficient for the application, there are several disadvantages to their use that are difficult to solve. Among these are:
(a) Metals that are inherently corrosion resistant, such as aluminum or stainless steel are generally more expensive than less corrosion resistant such as untreated carbon steel. As such, use of such materials can negatively impact the economics of a particular installation.
(b) While carbon steel is less expensive than aluminum or stainless steel, it subject to corrosion under normal atmospheric conditions. A common technique for improving the corrosion resistance of carbon steel is galvanization, which forms a layer of zinc alloy at the surface that acts as a sacrificial anode. Typically, galvanization can extend the life of steel in atmospheric conditions to 30 years or more. However, the galvanizing process increases the cost of the raw material significantly. Furthermore, the galvanizing process is generally considered to be environmentally hazardous, and as such the number of domestic suppliers has been in recent decline.
(c) Because even the most commonly available metals are generally expensive materials, it is desirable to design systems in such a way as to minimize their use. The minimization of material use tends to lead to tracking apparatus structure designs that are often complex. The manufacturing processes required to build components for complex systems are generally not suitable for performance in the field. As a result, components are usually manufactured and often assembled at locations remote to the final installation site. This can result in increased shipping costs.
Reinforced concrete is a common material used in a wide range of structures, from buildings to bridges to utility poles. As such, the formulation, production, forming, placing, and handling of reinforced concrete is extremely common and well known in construction practice. Although not found in the prior art, the use of reinforced concrete in tracking apparatus structures in addition to its use in prior art as a foundation material is potentially advantageous. This is due in part to the much lower cost of reinforced concrete on an equivalent strength basis when compared to the aforementioned metals currently used for such structures. Reinforced concrete is also inherently corrosion resistant in normal atmospheric conditions. Furthermore, reinforced concrete can be molded into a wide range of shapes, which allows some optimization of structural elements for a specific application.